Next-to-last stage buckets for turbines are typically connected to one another in groups with cover or shroud bands at the tip and a loose tie wire at about the mid-point of the buckets. Unfortunately, under certain conditions, grouped bucket designs such as these can be stimulated by dynamic steam forces and vibrate at the natural frequencies of the grouped buckets and cover assembly. If the vibration is sufficiently large, fatigue damage to the bucket material can occur and lead to crack initiation and eventual bucket failure. Further, the loose tie wire connection requires a hole or opening in each bucket which can lead to high centrifugal stresses at the hole and greater susceptibility to stress corrosion or fatigue cracking at the hole.
The grouped bucket design also results in gaps between the ends of adjacent bucket cover groups. These gaps permit steam leakage at the tip between cover groups and can reduce the thermodynamic efficiency of the next-to-the-last stage.